Water treatment facilities are indispensable to the purification of potable, used and industrial water, wherein the water has been exposed to contaminants of various size and composition. The purification process is thus intended to remove those contaminants with the use of appropriately selected methods, which are generally relying on the containment of water in large tanks in order to apply the treatment. Some contaminants are dense enough to sink and accumulate at the bottom of those tanks, depending of the flow rate, while others are big enough to be successfully sieved from the water with a filter. However, some contaminants, called colloids, are microscopic particles evenly distributed inside a mixture that cannot be separated effectively from the hydrocolloid solution, which is the water and colloid mixture, by physical means and thus require specific treatment methods.
In order to separate the water from those unwanted pollutants, it must go through certain steps of purification. Pre-treatment can be made in order to retrieve large debris and adjust the pH of the water to facilitate ulterior steps of the treatment. To eliminate the smaller particles in suspension and thus clarify the water, water treatment facilities generally comprise a flocculation zone where a flocculating agent, either a polymer (like modified polyacrylamides), a chemical product (like sodium silicate) or in rare occasions a natural product with the same properties, is introduced within the water. With the addition of such a flocculating agent, flocs particle aggregates) of contaminants start to form out of the colloids. A mixer with rotative blades generally stirs the mixture located inside the flocculation zone in order to maximise the contact between the flocculating agent and the contaminants, thus enabling the creation of bigger flocs.
This first step process, called flocculation, can be further improved with the addition of a ballasted material, like micro-sand, which acts as ballast and contact mass catalyzing the flocculation reaction inside the water and contaminants solution. When ballast is added, the aforementioned flocculating agent bonds it together with flocs of colloids and other particles, thus creating even bigger and heavier flocs by agglomerating previously created flocs along with sand particles. This in turn has the advantage of making the flocculation and the next step of the treatment happen faster.
The next step of the water treatment process is called sedimentation. It takes place in the sedimentation zone and capitalizes on the fact that gravity pulls every object toward the surface of the earth with a force proportional to its weight. Therefore, heavier particles are more easily dragged toward the bottom of this containment zone so the addition of granular ballast like sand, while not essential, can make a worthy addition to the process, reducing the time needed for the flocs to settle down at the bottom of the zone. The flocculation process is thus essentially a means of reducing the amount of colloids in suspension inside the liquid solution, creating relatively heavy flocs out of colloids which do not effectively sink to the bottom of the sedimentation zone with the influence of gravity as would the bigger particles in suspension inside the liquid solution. Purified water is subsequently collected when it overflows from the sedimentation zone. If ballast is used in the flocculation zone, ballasted flocs then accumulate at the bottom of the sedimentation zone and comprise both sand and particulate contaminants, further requiring to be treated to separate the sand from the pollutants.
The mixture comprising contaminants, colloïds, water and also sometimes sand form what is generally called “sludge”, which is to be removed from the system after the extraction of as much of the sand and water as possible in order to maximize the efficiency of the process. The extracted sand can be used again and again in the process without the need to add much more throughout the course of action, depending on the effectiveness of the aforementioned extraction.
A non-essential additional step, called coagulation, can be added to the water treatment process in order to further improve its efficiency. If included in the process, it is generally the first step by which the polluted water begins its purification after pre-treatment. It consists in the addition of trivalent metallic salts to the water and contaminants solution. The salts (generally iron or aluminium composites) dissolve in water releasing ions with three positive charges which bind with colloids and then form small aggregates. Those aggregates are combined into flocs when a flocculating agent is added to the solution and because they are bigger particles than the colloids themselves, they make the agglomeration of aggregates into flocs relatively easier than the process without prior coagulation and thus augment the efficiency of the procedure at the cost of the inclusion of another zone to the facilities, the coagulation zone, and added expenses for the trivalent metallic salts.
The purified water is generally filtered after the sedimentation zone in order to remove unsettled flocs and particles which could still be in suspension inside the water. Water concentration of the sludge produced after sedimentation is still too high and thickening means are therefore needed to reduce it enough to facilitate transport, for example to landfill sites. This added process takes a lot of time to be efficient and often necessitates large amounts of space, as in the case of open air evaporation sites (or drying beds). An alternative is the method of pressing which requires the sludge to be pressed against textile filters to extract as much liquid as possible after what a compact residual cake is made out of the remaining solid contaminants. The method of centrifugation uses centrifugal force to extract water from the sludge, and as for pressing the residual contaminants are shaped in a compact cake. On the other hand, these methods require specialized machinery or vast open spaces to be efficient, which are costly and may be impractical depending of the economic and geographic situation of the community requiring them.
Another common problem of actual water treatment facilities is the extraction of sand ballast from the produced sludge which results in needless waste of material.